Mr Spicer told Fox News: "I was trying to describe the attack on his own people - the use of chemical weapons.

"Frankly, I mistakenly made an inappropriate and insensitive reference to the Holocaust for which, frankly, there's no comparison.

Video:17 Mar: Sean Spicer repeats claims against GCHQ

"Obviously, that's not what I wanted to do and especially during this week, I regret using that term.

"I apologise and hope that we continue to focus on the President's decisive action that he took to deal with the situation in Syria."

When comparing Mr Assad's actions to those of the Nazi leader, Mr Spicer told reporters during a news conference that Hitler "didn't even sink to using chemical weapons".

Millions of Jews were killed in concentration camp gas chambers during the Holocaust.

Video:10 Mar: Sean Spicer wears his US flag pin upside-down

The backlash had begun on social media even before Mr Spicer had completed the briefing.

Steven Goldstein, executive director of the Anne Frank Center for Mutual Respect, said: "On Passover no less, Sean Spicer has engaged in Holocaust denial, the most offensive form of fake news imaginable, by denying Hitler gassed millions of Jews to death.

"Spicer's statement is the most evil slur upon a group of people we have ever heard from a White House press secretary.

"Sean Spicer now lacks the integrity to serve as White House press secretary and President Trump must fire him at once."

Video:7 Feb: Has Trump given us a golden age for comedy?

The Democrats also called for Mr Spicer to be sacked, with party leader in the House Nancy Pelosi saying in a statement: "While Jewish families across America celebrate Passover, the chief spokesman of this White House is downplaying the horror of the Holocaust.

"Sean Spicer must be fired, and the President must immediately disavow his spokesman's statements. Either he is speaking for the President, or the President should have known better than to hire him."

In another faux pas, Mr Spicer misspoke during an interview with CNN, saying Mr Trump was trying to "destabilise" the Middle East. He later clarified that the US wanted to "bring stability" to the region.

He had been answering questions about last week's attack in northern Syria, which left nearly 90 people dead after sarin gas was allegedly used.

Video:Jan 23: White House - 'Our intention is not to lie'

The US has blamed Mr Assad and said he acted with Russia's support.

Mr Trump's White House faced accusations of anti-Semitism just days after he took office when a statement marking International Holocaust Remembrance Day neglected to mention Jews.